Bellcore Technical Reference TR-NWT-001112, Issue 1, June 1993, entitled "Broadband ISDN User to Network Interface and Network Node Interface Physical Layer Generic Criteria", defines both the user-to-network interface and network node interface as bidirectional, as do similar ANSI/ITU standards. Similarly, ANSI T1.105-1991, entitled "Digital Hierarchy-Optical Interface Rates and formats Specifications (SONET)" assumes bidirectional links having equal bandwidth. Although ports defined according to these criteria are defined as bidirectional, the use of any such standard links such as asynchronous transport mode (ATM) for video distribution services can be significantly cost-reduced by means of unidirectional ports, since the bandwidth downstream is much greater than needed in the upstream direction. The cost reduction results from the savings of not having to provide an optical transmitter, i.e., laser, and receiver on each video link, as well as the savings of a fiber cable. The question, however, is how to monitor the quality of the video signal being distributed to downstream subscribers without the defined bidirectional monitoring scheme inherent in the DS3, SONET and ATM standards.